The present invention relates to an apparatus for measuring the velocity of flow of fluids, and particularly of the intake air of internal-combustion engines, having an electrically heatable resistance layer which is applied in insulated manner on a support and is in thermal contact with the flowing fluid.
Sensors for measuring the mass of air drawn in are required, in particular, for the control of internal-combustion engines. In known sensors a resistance layer which is applied to a flat support is traversed by a current, this resulting in heating. Depending on the velocity of flow of the air flowing around the sensor, a larger or smaller amount of heat is withdrawn from the resistance layer and support. By the use of a resistor material of temperature-dependent resistance the current can be regulated in such a manner as to obtain a constant temperature above that of the air. The current required for this is then a measure of the velocity of flow.
A number of, in part, contradictory requirements are made on such sensors. Thus, for instance, good heat transfer between the flowing fluid and the resistance layer is necessary for a sufficiently high sensitivity of response. In order to assure this, the flowing fluid is moved past on both sides of the resistance layer or support, which, however, is only suitably effective if the support is of good thermal conductivity and/or is correspondingly thin. High thermal conductivity of the support material results in a rapid equalization of temperature on the support, which favors a rapid response time.
Furthermore, the thermal and mechanical stability of such sensors must satisfy high demands. Finally, economical manufacture is necessary, to which simple assembly contributes.